A transformer is often used to step up or step down voltage and usually consists of one or more windings wound on a magnetic core. During operation, electrical energy is transformed into heat energy due in large part to eddy currents and hysteresis losses. Excessive heating of a transformer can cause adverse results, such as reduced efficiency and damage to the transformer. During low frequency use under 400 Hz., most of the heat is produced in the core of the transformer. However, at higher frequencies above 400 Hz., losses in the core decrease due to the smaller magnitudes of eddy currents. At the same time, however, heat is produced in the windings due to I.sup.2 R losses and skin effect. The heat produced in the windings increases with frequency and may cause fatigue and destruction of the windings or may adversely affect other components in the proximity of the transformer. Also the windings must have a large diameter and must be overrated to withstand the heat produced.
The prior art has disclosed attempts to cool transformers or parts thereof. However, the prior art devices are not entirely satisfactory for cooling transformer windings during high frequency use.
German Patent No. 2,218,659 discloses a cooling system which includes multiple axial cooling channels disposed concentrically around a transformer core. These channels run parallel to one another and are disposed between groups of concentric windings. The parallel channels are formed by wrapping the windings on coaxial formers of increasing diameter that are placed around the core and supported radially by spacers. The windings are disposed within the cooling channels themselves. Fans blow cooling air through these parallel channels to cool the windings. Because the windings are within the cooling channels, only coolants which do not react with the insulation of the windings can be employed. This system also increases the size of the transformer as there must be space between each concentric group of windings for the passage of air. In addition, since the coaxial formers completely encircle the core, they may undesirably form secondary windings.
Swiss Patent No. 249,488 also appears to disclose several non-enclosed axial cooling channels disposed concentrically around a transformer core which is disposed in an oil bath. These channels run parallel to one another and are formed between groups of high voltage windings. These non-enclosed channels expose the high voltage windings to the coolant and thus limit the type of coolant to ones which do not react with the winding insulation. These channels also are only able to cool the high voltage windings surrounding the channels and not low voltage windings wrapped about the high voltage windings.
Wadhams, U.S. Pat. No. 2,547,065, discloses a transformer cooling system consisting of hollow cooling plates through which coolant passes. These plates are located between the laminations of a transformer core. This system, however, would be inefficient when used to cool sets of transformer windings having a great number of turns since only the innermost windings closest to the cooling plates could be cooled.
Sabol, U.S. Pat. No. 2,547,045, also discloses a first cooling system consisting of cooling plates between core laminations of a transformer. The edges of these plates contain tubing for the passage of coolant. A second cooling system disclosed by Sabol includes tubing attached externally to legs of the core. Both these systems, like that disclosed in Wadhams, would be inefficient when used to cool the windings of a transformer having a great many turns.
Burgher et al., U.S. Pat. No. 4,577,175, Dunnabeck et al., U.S. Pat. No. 3,144,627 and Strickland, U.S. Pat. No. 2,577,825, all disclose cooling systems where at least a portion of a winding is formed from a tubular member through which a fluid coolant passes. These cooling systems are not practical, however, for a transformer that requires windings having many turns since the tubular member would occupy a large volume, causing the resulting transformer to be unduly large.